rgw/pubsub: return 404 NotFound instead of NoSuchKey

rgw/pubsub: return 404 NotFound instead of NoSuchKey

repurpose the ERR_NOT_FOUND define which was otherwise unused to
customize the error response for sns apis, which return the NotFound
error code instead of NoSuchKey from s3:

    https://docs.aws.amazon.com/sns/latest/api/API_GetTopicAttributes.html#API_GetTopicAttributes_Errors

this allows boto3 sns clients to catch the NotFoundException as expected

Signed-off-by: Casey Bodley <cbodley@redhat.com>